The invention relates to a device and to a method for controlling the position of a tape within a tape transport system by calibrating units of the tape transport system considering the spring constant of the tape, dependent on the number of unsupported wraps caused by air entrainment.
In modern tape storage systems, the capacity and performance of the tape storage systems have increased considerably. To achieve higher cartridge or tape capacities and improved performance, however, further advances in several areas are necessary. Increases in linear and track densities on the tape may be required in order to achieve higher storage capacities. However, increase in linear densities may lead to a decrease of the distance between adjacent bit cells, which in turn may lead to an increase of inter-symbol interference. Increase in track densities may lead to narrower individual track widths and narrower write and/or read heads, which may require a very precise control of the tape transport system and track-follow control of the tape head. Thus, reliable and precise tape transport is of importance to guarantee read-channel performance on all parallel data channels during tape operation.
An increase in volumetric density may be enabled by the use of thinner tape material. In this case, to achieve a reliable and precise tape transport, tight control of tension and potentially of tape dimensional stability (TDS) variations may become necessary. Usually, the performance of the tape transport servomechanism and the quality of readback signals in data channels are affected by variations in the tape velocity and tension. For instance, during operation in cruise velocity mode, periodic variations of tape velocity and tension around the nominal value, also called once-around, may be induced by reel eccentricities. This problem may become critical when the reel rotation frequencies are near the resonance frequency determined by the tape path.
Conventionally, for the tape transport operation, a dual servo channel provides estimates of the tape velocity, tape longitudinal position, and head lateral position, which are derived from servo signals that are obtained by servo readers in the head module reading pre-formatted servo information on tape. In tape transport control systems, the tape velocity measured at the head using pre-formatted servo information, the so-called primary velocity, and secondary velocities are used for a velocity control during cruise mode. Hall sensors can be used to obtain the secondary tape velocity information from the individual reels, which information is typically used to achieve proper tape transport operation in the absence of valid parameter estimates from the servo channel.
However, during operation in cruise velocity mode, variations of tape tension around the nominal value may be induced by reel eccentricities leading to tension variations and therefore time-varying fluctuations of the individual data track widths and time varying spacing between individual data tracks, as the tape is stretched differently depending on the actual tape tension. In tape transport, this problem may be particularly serious when the reel rotation frequencies are near the resonance frequency determined by the tape path. Tape tension errors may also affect the position error signal used for track-following control, and hence the performance of the track-following servo system which aligns the head with the tape.
Therefore, a proper operation of a tape transport control system may require the knowledge of parameter values that determine the characteristics of the tape transport system. A system for operating a reel-to-reel system, which uses information in view of tension disturbances, is for example disclosed in US 2011/0134562 A1.